Production of carbonyl iron powder



Dec. 4, 1956 DE wlTT H. wEsr ET AL. A2,772,956

PRODUCTION OF CARBONYL IRON POWDER Filed June 24, 1955 'Patented Dec. 4, 1956 PRODUCTION OF CARBONYL IRON POWDER De Witt Henry West, Port Eynon, and William- Thomas Phelps, Swansea, Wales, assignors to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware Application June 24, 195s, senat No. 517,876

Claims priority, application Great Britain June 28, 1954 s claims. (Cl. 7s-.s)

The present invention relates t-o an iron powder and to aprocess for producing the iron powder and, more particularly, to .a special carbonyl-iron powder of improved electromagnetic properties: for use in magnetic cores and to a process for producing the special carbonyl iron powder.

It is wellV known that iron powder produced by the decomposition of ir-on pentacarbonyl and containing `both carbon and nitrogen is used for making magnetic cores.

Its suitability for this purpose is indicated by the Q valueV of a standard core made from the powder. As is well Vknown in the art, for optimum core eiciency it is essen- Vtial that the iron powder employed in the manufacture of a core assure a high Q value which is defined for a core coil assembly as the ratio of 21|- fL (inductive reactance) to R (series resistance), wherein f is the frequency, L is the inductance measured in henries and R is the resistance measured in ohms. It is desirable that Vthe inductance of the core assembly be as high as possible in contra-distinction to the resistance which should be as low as possible so that the optimum Q values can be obtained. The actual Q value depends on the size and shape of the core tested, so the Q values obtained are relative. The Q values for differentpowders must all be obtained on cores of `one size and shape if a proper comparison of the' effectiveness of the different powders is to be made. The Q values quoted in this specification have all been obtained with cores manufactured accord-- ing to a standard core specification, as follows. The powder is moistened with sodium silicate solution (15 Tw.), vdried to give 0.8% added sodium silicate. After gentle grinding to pass a 60 mesh sieve the insulated powder is mixed with 3% -of Bakelite powder, moistened with acetone and the mixture pressed at 45 tonsper sq. in. to give a core measuring 1/z diameter; after curing at 140 C. for 2 hours the core is ready for testing on a standard Q meter.

The principal types of powder produced at present are commonly known by grade names, as follows:

Y Grade L Powder of-meaniparticle sizeof about- 4:5 to about 5 microns containing `y`'about 017% carbon and 0.5% nitrogen. It is used for cores working at frequencies up to about 20 rnc/s. (megacycles per second). The Q values :obtained by testing cores made from.. this grade of powder are about: 167 to 169 at 20mm/s;

Grade 11p-Powder of mean particle sizeof about 3' to about 3.5 microns containingV from 0.5f to 0.6% carbon and 0.5 to 0;6% nitrogen. This powder is used for cores operating at higher frequencies. The Q- values of cores madeof grade II powder are about 190 at 2() mc./s., about 167 at 50'mc./s., and about 85 at 200 mc./s.

Grade IIl.-Powder of mean particle sizeof about 6T microns containing less than 0.1%A carbon together with about 0.01% nitrogen. This powder is used for cores working at low frequencies when the permeability must be high. p

VIt has been proposed in the manufacture of carbonyl iron` powder to add small quantities of ammonia to the tion.

gas fed to the decomposer to assist in the decomposition of the iron carbonyl. The standard form of decomposer is a cylindrical vessel into which the gas feed is introducedat the top, the powder being removed from the bottom. This vessel is surrounded by a heating jacket throughv which hot gases flow. The powder produced is the aforementioned grade l. The grades II and III powders are made by subjecting this grade I powder to gaseous elutria- In the elutriation the finest particles, usually amounting to about 20% by weight, are separated. These fine particles constitute' the aforementioned grade II powder. about 80% of the grade I powder, has a coarser mean particle size than grade I. This relatively coarse residual portion is converted to the aforementioned grade IH by treatment in converted to the aforementioned grade III by treatment in hydrogen. Thus, grades II and III are produced by the foregoing procedureY in a substantially constant proportion, which is notalways required but is an inevitable result of using elutriation. The separatio-n process is inconvenient and the art has endeavored to produce very fine powder in the thermal decomposition operation. lf the temperature throughout the whole decomposer is increasedA the nitrogen content may be increased while the particle size of the product is simultaneously decreased. However, if the particle size is de* creased unduly, diculty` is encountered in insulating such very line particles of powder from -o-ne another in the cores for the purpose of reducing eddy current losses. Although' many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as

i far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that the nitrogen content of the powder has an important inuenceon its electromagnetic properties and in particular on the Q values at high frequencies, and that the nitrogen content should be higher than that heretofore commonly present. further been discovered that .when controlled amounts of nitrogen are introduced into the carbonyl-iron powder solely during the production of the powder in the decorn poser the electromagnetic properties of the powder are far superior to those of an iron powder having a similar amount of nitrogen which has been introduced subsequent tothe carbonylldecomposition operation by heating the iron powder in ammonia and cooling it in nitrogen. In other words, the inventors haveV found that magnetic core material o f improved high-frequency characteristics can be directly produced in the carbonyl decomposer with the iron powder product havinghigher Q value and nitrogen content thanV heretofore attained. An important feature of the present invention is the discovery that, when a higher temperatureis maintained in a zone at the bottom ofthe decomposerfthan above that zone boththe nitrogenv content and the Q values -Uo-f the powder, particularly the controlled range' of particle sizes directly in the thermalA ydecomposition operation.

The invention also contemplates providing an improved process of producing a carbonyl-iron powder yof' tine particle sizes without any gradiugor separation' operation.

VIt is a further object ofthe invention to provideanv irnproved process of producing a carbonyl-iron powder of high Q value .andgoo'd permeability at high frequencies of the order -of 20.' to 50 megacycles per second* and Vabovelin an improved carbonyl-decomposition furnace;V

The residual portion from the elutriation, usually- It hasY The invention further contemplates providing a carbonyl-iron powder having improved electromagnetic properties at high frequencies of the order of 2O to 50 megacycles per second and above.

It is 'another object of the invention to provide a magnetic core characterized by high Q values when employed at high frequencies of the order of to 50 megacycles per second and above.

Other objects and advantages will become apparent from the following description taken in conjunction with `the accompanying drawing which is a central, vertical, longitudinal section of a carbonyl decomposer in which the improved carbonyl-iron powder can be produced.

Generally speaking, the present invention contemplates `an improved process for producing carbonyl-.iron powder, of controlled nitrogen content and of a controlled particle size range, directly in the carbonyl decomposer by a special therm-al control therein, whereby the novel iron powder is characterized by high Q values, especially at high frequencies of the order of 20 to 50 megacycles per second and above, together with Vgood permeability. The improved results of the invention lare achieved when substan- Itial and controlled amounts of ammonia are introduced into the decomposer along with ironcarbonyl vapor and the temperature of the decomposer is regulated so that the decomposition of the iron carbonyl is carried out with a bottom zone of the decomposer at a higher temperature than above that zone.

In order to obtain the results of the inven-tion, ammonia is .introduced with fthe iron carbonyl vapor into a decomposer operated in such a manner that the bottom zone of the decomposer is maintained at a temperature between about 370 C. and about 500 C., and the portion of the decomposer labove that zone is operated at lower temperatures of at least about 250 C. and up Ito about 320 C., respectively. The amount of ammonia used should be at least about 15% and should not exceed about 100% by volume of the incoming iron carbonyl vapor feed. The iron carbonyl containing gas fed into the decomposer may be diluted with carbon monoxide in a proportion of from about to about 100% by volume lof the iron carbonyl content of the gas, if desired. It has been found that if the ammonia so introduced is not less than about 10% by volume of the iron carbonyl, the powder produced will have the .improved electromagnetic properties described herein, and more ammonia than about 100% by volume of the iron carbonyl tends to diminish the particle size of the product as well as being wasteful. The volume of any carbon monoxide included in the gas fed into the decomposer is not critical. However, it has been found desirable lto regulate the proportion of carbon monoxide at about 75% by volume of the iron carbonyl.

The composition of the gas feed can vary considerably. In producing grade I powder, a typical gas composition is about 13% ammonia, about 37% carbon monoxide, and about 50% iron carbonyl. To produce .the desired increased nitrogen contents of the powder according to the invention, it may be necessary to I'use a feed gas of higher ammonia content than heretofore used. However, increase in the volumetric ratio of ammonia to ir-on carbonyl beyond 1:1 is undesirable, since higher ammonia concentrations are wasteful `and also tend to keep down the particle size of the powder. In producing grade II powder by the new process the Iammonia content of rthe feed gas can be varied over a wide range, i. e. between about 15% and about 100% of the volume of the ingoing iron carbonyl vapour. The nitrogen content of the resulting powder will range from about 0.7% to about 2%. A suitable feed gas contains ammonia in a volume equal to that of the iron carbonyl vapor, making the composition: about 35% ammonia, about 30% carbon monoxide and about 35% iron carbonyl in percent by volume.

In carrying the invention into practice, it is advantageous -to use a decomposer of Vthe type shown in the drawing. This decomposer may be a cylindrical vessel 1 into Vwhich |the gas feed .is introduced through the gas inlet 3 at the top, and the powder is removed through the outlets 5 at the bottom, as is customary in the art. However, means are provided -to maintain the bottom zone at a higher temperature `than the portions of the dec-omposer above that zone. This means may consist of separate heating devices at the different zones of the decomposer, e. g. separate electrical heaters or separate gas heating jackets surrounding respectively the top zone T, the middle zone M and the bottom zone B of the decomposer. Asshown in the drawing, the cylindrical-decomposer vessel 1 is surrounded by a heating jacket 2. The portion of the heating jacket 2 surrounding the bott-om zone of the decomposer is separated from the remainder of the heating jacket by a partition 8 whereby the temperature of the bottom Zone of the decomposer can easily be maintained higher than that of the remainder of the decomposer. By this means the heating of the different zones of the decomposer maybe separately regu-lated to maintain these zones at appropriate temperatures for conducting the new carbonyldecomposition operation.

It has been found advantageous to use a decomposer of special proportions, i. e. the decomposer 1 has a greater ratio of height to diameter than the customary ratio of 3.5 :1. l

An illustrative example of the preferred method of carrying the invention into practice for producing grade I powder, the incoming gas contains about 35% ammonia, 30% carbon monoxide, and 35% iron carbonyl vapor. This gas is fed into the decomposer having a top temperature of about 290 C. and a bottom temperature of about 370 C. The powder produced by the foregoing process is sieved to remove undesirable aggregates and is then suitably milled. Such operations produce a novel iron powder having an average particle size of about 4.9 microns, a nitrogen content of about 1.5% and possessing a Q value of 187 at 2O mc./s. and 175 at 50 mc./s. A similar procedure may be followed for producing grade II powder, except that a top zone temperature of about 320 C. is used.

The value of increasing the bottom temperature of the decomposer is illustrated by the following results of tests shown in Table I. These tests were made on powders which were produced under otherwise identical experimental conditions, with a feed gas containing 35% ammonia, 30% carbon monoxide and 35% iron carbonyl. In the decomposer in which these powders were produced, the temperatures were measured at points 1A, 1/2 and 3A of the distance down the decomposer and situated approximately 1A diameter inwards from the walls as shown at 17, 18 and 19 on the drawing. The temperatures at the two upper zones T and M were about 290 C. in each case.

Table I Q value Temperature of Particle Nitrogen, Bottom Zone, C. Size Percent 20 mc./s. 50 mc./s.

It will be seen that the Q value increases apprecably with higher bottom temperatures without appreciably affecting the particle size of the powder.

By increasing the temperature in -the upper zone, the particle size may be reduced, but the temperature in the upper zone is not wholly independent of that in the bottom zone. Therefore, as the bottom temperature is increased the top zone temperature should be suitably decreased, if the mean particle size of the powder produced is to be maintained constant. In general, this decrease involves a minor adjustment of a few degrees as compared with the large temperature diierential between the temperatures of the top and bottom zonesin accordance withthe present` invention. The effect of varying the top zone temperature on the particle size of the powder at constant temperature ofA the bottom zone (370 C.) and with the same feed gas is shown by the test results in Table II.

The last powder has, a', particle size that is typical of grade II powder.

The necessity for still maintaining a high bottom temperature isA illustrated by the test results shown in' Table III. These testswere made onk powdersV produced. under otherwise identical experimental conditions, using the same feed gas as used for the tests made for Table I.

Table lII Zonetemperatures, QValues C. Particley Nitrogen,

Size Percent (microns) Upper Bottom 20 mc./s. 1 50 mc./s.

Table IV Particle Q Values Nitrogen, Percent size (microns) 20 mc./s 50 ine/s.

Event greater improvement, especially at the higher frequencies, is brought about by raising the bottom temperature still further. This is shown by the figures in the following table which were obtained with a feed gas composed of 47% iron carbonyl, 41% carbon monoxide and 12% ammonia. This table also shows how the tcmperature of the top zone may be reduced as the temperature of the bottom zone is increased if the particle size is to be maintained constant.

While the inventors .have found that the Q value of carbonyl-iron powder at high frequencies `can be increased independently of the particle size by raising the decomposer temperature, suitable adjustment of the composition of thel decomposer feed,l and contro1lingthe` nitrogen content of the carbonyl-iron powder produced, in such a procedure -for any decomposer temperature there is a critical nitrogen content at which the maximum Q value occurs. This critical nitrogen content appears to vary with the frequency at which the Q value is determined. As is well known, there is an upper limit beyond which the temperature of the decomposer can not be raised without producing unduly fine powders. However, the

inventors have further discovered that when the decomposition is carried outrwith the bottom of the decomposer at a higher temperature than the middle and top zones the powder produced has a' higherQ value and' nitrogen content than a powder made with the whole of Ithe decomposer at substantially the upper zone temperature, and without any decrease 4in the particle size and consequent decrease in permeability. Y When the ybottom zoneV is at ahigher temperature thanl the top, zone of the decomposer, the Q value of the powder is increasingly higher as the nitrogen content increases up to a maximum, and-then remains cons-tant as the` nitrogen content is further increased. Thus, the aforementioned critical correlation of nitrogen content-and decomposer temperature is no l'on'ger necessary toget the best results.

The carbonyl-iron powder prepared in accordance with thel process of this invention has a controlled range of particleV sizes. By appropriately regulating the operationl of the newprocess,` both grade I and grade II carbonyl-iron powdersl can be produced separately in the. decomposer inl` any quantity required. The powders produced by the new process have improved Q` values, especially at the higher frequencies; By varying thel upper.' zone temperature powder may be produced of* a grade intermediate between grades I and II or even finer thangra'd'eII.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for producing directly in the decomposition space of a carbonyl decomposer a carbonyl-iron powder characterized by a substantially uniform particle size and high Q values at high frequencies of the order of about 20 to 50 megacycles per second and above comprising feeding into the decomposer iron-carbonyl vapor, carbon monoxide, and effective amounts of ammouia in a volumetric ratio of ammonia to iron carbonyl of up to about 1:1, so heating the decomposer that the bottom zone thereof is maintained at a higher temperature than the portions of the decomposer above the bottom zone, the temperature of the bottom zone being between about 370 C. and 500 C., and controlling the grade of carbonyl-iron powder produced by regulating the temperature of the upper zone of the decomposer at a temperature lower than the temperature Table V Temps. C.) Q values Particle Percent size N2 in Top Bottom (microns) powder 50 mc./s. 100 mc./s. 150 mc./s. 200 mals. zone zone 7 of the bottom zone and within the range of from about 270 C. to about 320 C., the ineness off the carbonyliron powder produced increasing proportionately as the temperature of the upper zone of the decomposer is increased within the foregoing upper zone temperature range.

2. A process for producing directly in the decomposition space of a carbonyl decomposer a carbonyl-iron powder characterized by a controlled range of particle size and high Q values at high frequencies of the order of about 20 to 50 megacycles persecond and above comprising feeding into the decomposer iron-carbonyl Vapor, carbon monoxide in atproportion of from about 25% to Vabout 100% by volume of the iron carbonyl vapor, and effective amounts of ammonia in a proportion of from about 15% to about 100% by volume of the iron carbonyl vapor; so heating the decomposer that there are superimposed zones of different temperature, with the bottom zone thereof maintained at a higher temperature than the top zone, and the temperature of the bottom zone being between about 370 C. and 500 C.; and controlling the grade of carbonyl-iron powder produced by regulating the temperature of the top zone of the decomposer at a temperature lower than the temperature of the bottom zone and within the range of from about 250 C. to about 320 C.

3. A process for producing directly in the decomposition space of a carbonyl decomposer a carbonyl-iron powder characterized by a substantially uniform particle size and high Q values at high frequencies of the order of about 20 to 50 megacycles per second and above comprising feeding into the decomposer iron-carbonyl vapor, carbon monoxide and effective amounts of ammonia in a proportion of from about 10%V to about 100% by volume of the iron carbonyl vapor so heating the decomposer that the bottom zone thereof is maintained at a higher temperature than the top zone of the decomposer, the temperature of the bottom zone being 8 between about 370 C. and 500 C. and the temperature of the top zone being between about 250 C. and 320 C.

4. A process for producing directly in the decomposition space of a carbonyl decomposer a carbonyl-iron powder characterized by a substantially uniform particle size and high Q values at high frequencies of the order of about 20 to 50 megacycles per second and above comprising feeding into the decomposer iron-carbonyl vapor and effective amounts of ammonia; so heating the decomposer that the bottom zone thereof is maintained at a higher temperature than the portions of the decomposer above the bottom zone, the temperature of the bottom zone being between about 370 C. and 500 C. and the temperature of the top zone being between about 250 C. and 320 C.

5. A process for producing directly in the decomposition space of a carbonyl decomposer a carbonyl-iron powder characterized by aV substantially uniform particle size and high Q values at high frequencies of the order of about 20 to 50 megacycles per second and above comprising feeding into the decomposer iron-carbonyl vapor and substantial amounts of ammonia; so heating the decomposer that a higher temperature is maintained in a bottom zone thereof than above that zone, the temperature of the bottom zone being within the range of about 370 C. to 500 C. and the ltemperature. of the top zone being within the range of about 250 C. to 320 C.; and controlling the grade of carbonyl-iron powder produced by regulating the upper zone temperature inversely to the bottom zone temperature in said ranges to maintain substantially constant the mean particle size of the powder produced.

References Cited in the le of this patent Carbonyl Nickel and Carbonyl Iron Powders, Their Production 'and Properties. B. I. O. S. Report 1575. (Interrogation Report 590), item No. 21, page 21. 

1. A PROCESS FOR PRODUCING DIRECTLY IN THE DECOMPOSITION SPACE OF A CARBONYL DECOMPOSER A CARBONYL-IRON POWER CHARACTERIZED BY A SUBSTANTIALLY UNIFORM PARTICLE SIZE AND HIGH Q VALUES AT HIGH FREQUENCIES OF THE ORDER OF ABOUT 20 TO 50 MEGACYCLES PER SECOND AND ABOVE COMPRISING FEEDING INTO THE DECOMPOSER IRON-CARBONYL VAPOR, CARBON MONOXIDE, AND EFFECTIVE AMOUNTS OF AMMONIA IN A VOLUMETRIC RATIO OF AMMONIA TO IRON CARBONYL OF UP TO ABOUT 1:1, SO HEATING THE DECOMPOSER THAT THE BOTTOM ZONE THEREOF IS MAINTAINED AT A HIGHER TEMPERATURE THAN THE PORTIONS OF THE DECOMPOSER ABOVE THE BOTTOM ZONE, THE TEMPERATURE OF THE BOTTOM ZONE BEING BETWEEN ABOUT 370* C. AND 500* C., AND CONTROLLING THE GRADE OF CARBONYL-IRON POWDER PRODUCED BY REGULATING THE TEMPERATURE OF THE UPPER ZONE OF THE DECOMPOSER AT A TEMPERATURE LOWER THAN THE TEMPERATURE 